Prior art discloses only amplifiers with a load at their output (FIG. 1), but does not disclose amplifiers with a load at their input (FIGS. 2, 3 and 4) directly connected to the power supply.
Different embodiments of switching class-D amplifiers for a mono-phase load (for instance, single voice coil loudspeaker) are disclosed in the following U.S. Pat. Nos.: 3,585,517 issued June 1971. to R. B. Herbert (FIG. 6); U.S. Pat. No. 4,649,565 issued March, 1987. to A. J. M. Kaizer et al. (FIG. 5); and Re. 33,333 issued Sep. 11, 1990. to W. E. Taylor, Jr. et al.
Class D amplifier for a two-phase load (for instance, a loudspeaker with two grounded voice coils) is disclosed in U.S. Pat. No. 4,360,707, issued November 1982. to J. R. Joseph et al.
Output LC filter of a class D amplifier, used for reconstruction of the amplified signal at the output of the switching bridge, requires a large number of components of significant size, whereby the price and the dimensions of class D amplifier are considerably increased.
If the amplifier load is other than nominal for output LC filter, the amplitude response will significantly depart from the designed one. In case of a load value less than nominal, the amplitude response will be less than that designed. In case of a load value greater than nominal, amplitude response will be greater than that designed. In case of a unloaded amplifier operating in the vicinity of parallel resonance frequency of the output LC filter, extremely high voltage is generated, which could lead to a filter capacitors breakthrough.
However, the majority of loads used nowadays, such as electrodynamic loudspeakers, induction electric motors and brushless DC motors, are characterized by a significant inductance of their windings in relation to resistance, so that they are completely unfit for direct connection to output LC filters which are designed for a purely resistive load. One skilled in the art solves this problem by the use of a Sobel filter connected in parallel to the inductive load, whereby the total impedance becomes purely resistive at all frequencies of interest. Sobel filter consists of a resistor of the same value as the load resistance, serially connected to a specially selected capacitor canceling the effect of the load inductance. However, this solution considerably increases dissipation of the switching bridge because additional low impedance is connected in parallel to the amplifier output.
Output LC filters of audio amplifiers and fast reacting electric motor drives feature relatively low impedance of filtering inductances, leading to an increase in current ripple through all transistors in the switching bridge, and thereby to increased dissipation on them and resistances of filter inductances.
Input LC filter of a class D amplifier, which is used to reduce injection of conducted EMI noise from the switching bridge into the cable connecting the power supply, requires bulky components, whereby the price and the dimensions of class D amplifiers are increased.
The design of such input LC filter requires special attention due to mutual interaction between its output impedance and the input impedance of the switching bridge, in order to avoid voltage oscillations at the switching bridge.
A special problem appearing during the operation of class D amplifiers with both positive and negative power supplies is power supply bus “runaway” during the amplification of low frequency signals. During the positive half cycle of an input signal, the observed switch is on most of the time and power is delivered to the load and partly accumulated in the filter inductor. During the negative half cycle, the observed switch is off most of the time, and the current of the filter inductor returns back to the power supply through the diode antiparallel to the observed switch. In that case, during the positive half cycle the positive power supply voltage is decreased, while during the negative half cycle it is increased. Bearing in mind that most power supplies are made to source, and not to sink the current, the voltage increase requires utilization of bulky capacitors or special protection circuits in the power supply.
More detailed discussion of the problems associated with state of the art class D amplifiers is given in application notes: AN1042 “High Fidelity Switching Audio Amplifiers Using TMOS Power MOSFETs” issued by Motorola Semiconductor in 1989., AN1013 “Mono Class-D Amplifiers” issued by SGS-Thomson Microelectronics in 1998., AN9525 “Class-D Audio II Evaluation Board” issued by Harris Semiconductor in 1996., SLOU032A “TPA005D02 Class D Stereo Audio Power Amplifier Evaluation Module User's Guide” and “A Real Analysis of the Power Behind Audio Power Amplifier Systems” both issued by Texas Instruments in 1998.
A standard high-power amplifier consisting of a switching power supply for voltage increase (boost converter) (FIG. 8) connected to a class D amplifier (FIG. 9), as well as its modified version, i.e. a switching amplifier made of two switching power supplies for the voltage increase and a mono-phase load at its output (FIG. 10), are disclosed in U.S. Pat. No. 4,186,437 issued January, 1980. to S. M. Cuk, and scientific paper: R. O. Caceres and I. Barbi, “A Boost DC-AC Converter: Analysis, Design, and Experimentation”, IEEE Transactions on Power Electronics, Vol. 14, No. 1, pp. 134–141, January 1999. While standard version is utilized in audio amplifiers, its modified version requires very small inductors and capacitors for the reproduction of audio frequencies, leading to increased current ripple through all switches and increased output voltage ripple, significantly reducing amplifier efficiency. Feedback is also complex and requires current sensing through inductors and voltage sensing at output capacitors. In case of any instability in the feedback loop, the output voltage is unlimited and could provoke switches breakthrough.
Linear push-pull amplifiers for a two-phase load (for example, a loudspeaker with two grounded voice coils) are disclosed in the following U.S. Pat. Nos.: 4,130,725, issued December 1978. to M. J. Nagel, U.S. Pat. No. 4,201,886, issued May 1980. to M. J. Nagel, and U.S. Pat. No. 4,220,832, issued September 1980. to M. J. Nagel.
Linear class AB amplifiers for a two-phase load (for example, a loudspeaker with two grounded voice coils) with variable voltage power supply are disclosed in U.S. Pat. No. 5,748,753, issued May 1998. to R. W. Carver.
The basic problem in all existing linear audio amplifiers in classes A, B and AB is the generation of heat and low efficiency during normal operation, requiring high power consumption from the power supply, which is of specific interest for battery supplied devices such as those in cars, portable computers, radios, cassette and CD players.
The problems of conducted and radiated EMI noise generation in spite of input and output LC filters utilization in switching amplifiers (inverters) for a three-phase load (for example, induction electric motor or brushless DC motor) are disclosed in U.S. Pat. No. 5,661,390 issued August, 1997. to T. A. Lipo et al. (FIG. 7), as well as in scientific paper D. A. Rendusara and P. N. Enjeti, “An Improved Inverter Output Filter Configuration Reduces Common and Differential Modes dv/dt at the Motor Terminals in PWM Drive Systems”, IEEE Transactions on Power Electronics, Vol. 13, No. 6, pp. 1135–1143, November 1998.